1. Field of the Invention
The invention relates to a method for producing terephthalic acid, more particularly to a method for producing terephthalic acid via a Diels-Alder reaction at a low temperature and in the absence of a solvent. The invention also relates to a method for producing methyl 4-methyl-3-cyclohexene-1-carboxylate, which is used for further producing terephthalic acid.
2. Description of the Related Art
Polyethylene terephthalate (hereinafter referred to as PET) is widely used in the industries for the manufacture of commodities, such as fibers, containers, and packaging materials, and purified terephthalic acid (hereinafter referred to as PTA) is one of the major monomer raw materials for PET.
Nazarov et al. (Russ. Chem. Bull. 1959, 8, 1362-1369) discloses a process for producing terephthalic acid by subjecting isoprene and methyl acrylate to a hydroquinone catalyzed Diels-Alder reaction in the absence of a solvent, followed by dehydrogenation, hydrolysis, and oxidation. However, the Diels-Alder reaction in the aforementioned process is very slow at low temperature (7 months at 20° C.). Therefore, it is necessary to provide an additional energy source for heating so as to accelerate the reaction (6 hours at 120° C., 2 hours at 200° C., and a flow system at 400° C.). Moreover, the selectivity for a para product is lowered when the reaction temperature is raised. Additionally, as oxidation is performed in the presence of chromium (VI) trioxide (as a catalyst), heavy metal contamination will inevitably cause waste water problem. Therefore, a final mixture of the terephthalic acid and isophthalic acid obtained from the aforementioned process needs to be further separated using other reagents.
TAKASHI et al. (J. Org. Chem. 1966, 31(4), 1121-1123) discloses a process for producing methyl 4-methyl-3-cyclohexene-1-carboxylate by subjecting isoprene and methyl acrylate to an aluminum chloride catalyzed Diels-Alder reaction in the presence of benzene. However, the aforementioned process involves not only costs of the consumption of an organic solvent such as benzene, but also problems of solvent pollution and recovery. Moreover, although in general product yield can be increased by using a solvent in a reaction, the product yield of the aforementioned process is only 50% after a 3-hour reaction at a temperature ranging from 10° C. to 20° C.